# Set the `value` at row `y` and column `x`
	#
	# Require: `x >= 0 and x <= width and y >= 0 and y <= height`
	#
	# ~~~
	# var matrix = new Matrix.identity(2)
	#
	# matrix[0, 0] = 0.0
	# matrix[0, 1] = 0.1
	# matrix[1, 0] = 1.0
	# matrix[1, 1] = 1.1
	#
	# assert matrix.to_s == """
	# 0.0 0.1
	# 1.0 1.1"""
	# ~~~
	fun []=(y, x: Int, value: Float)
	do
		assert x >= 0 and x < width
		assert y >= 0 and y < height

		items[x + y*width] = value
	end